A manufacturing method described in PTL 1 (Japanese Unexamined Patent Application Publication No. Sho57(1982)-151696) has been known as a method of manufacturing molded coal through pressure-molding of powdery material coal. In this manufacturing method, dried powdery molding material coal and a binder such as coal-tar pitch are blended, and such a blend is pressure-molded to produce molded coal. Specifically, in this manufacturing method, a water coating is formed on a mold surface of a molding machine by water spray or steam blowing, and then the blend is pressure-molded.
This manufacturing method described in PTL 1 is intended to prevent insufficient removal of molded coal from the mold. In the manufacturing method, therefore, a water coating is formed on the mold surface, thereby wettability of the binder to the mold surface is extremely lowered. Accordingly, adhesive strength of the binder is also lowered. As a result, insufficient removal of molded coal from the mold can be prevented.
In the manufacturing method described in PTL 1, however, the binder must be mixed in the powdery material coal for pressure molding, and thus lower molding cost has been an issue.
A manufacturing method described in PTL 2 (Japanese Unexamined Patent Application Publication No. 2010-116544) has been known as a method of manufacturing molded coal by pressure-molding powdery material coal with no binder.
The method of manufacturing the molded solid fuel described in PTL 2 includes a mixing step of mixing a mixed oil containing a heavy oil and a solvent oil with pulverized low-grade coal to produce a slurry, an evaporation step of heating and dehydrating the slurry to produce a dehydrated slurry, a solid-liquid separation step of separating the solvent oil from the dehydrated slurry to produce a cake, a drying step of heating the cake to further separate the solvent oil from that cake to produce powdery upgraded coal, a humidification step of humidifying the powdery upgraded coal to produce humidified upgraded coal having a water content of 3 to 10 wt %, and a molding step of pressure-molding the powdery, humidified upgraded coal by a double-roll molding machine to produce a molded solid fuel in a form of briquettes.
In the manufacturing method described in PTL 2, although a molded material having high strength is produced due to water addition in the humidification step, when a powdery coal having a low carbonization level and a low bulk density is molded, the strength improving effect is not sufficiently exhibited, leaving room for further improvement.
Specifically, in the manufacturing method described in PTL 2, the powdery upgraded coal to be pressure-molded has a high temperature of 100° C. or higher after the drying step. In addition, friction occurs between each roll surface of the double-roll molding machine and the powdery upgraded coal during the pressure molding by the double-roll molding machine. As a result, a surficial portion of the molded solid fuel has even higher temperature, leading to a large temperature difference between the surficial portion and the inside thereof. Hence, the molded solid fuel has internal strain due to the temperature difference, and is reduced in strength. In this regard, the manufacturing method described in PTL 2 leaves room for improvement.
In the manufacturing method described in PTL 2, water added in the humidification step evaporates from the surficial portion of the molded solid fuel during the pressure molding by the double-roll molding machine. As a result, a large difference in moisture percentage occurs between the surficial portion and the inside of the molded solid fuel. Hence, the molded solid fuel is reduced in strength due to the difference in moisture percentage. In this regard, the manufacturing method also leaves room for improvement.